Blowout preventers are used to maintain control of wells during drilling. Ram-type blowout preventers are used to close on the drill or pipe string to contain pressure in the well. At times it is necessary to strip the string through the closed rams. This stripping movement can severely wear or abrade the face of the resilient ram front packer. Another reason that the ram front packers of ram-type blowout preventers are subject to wear is that in order to provide a seal about the string they must move into tight engagement with the irregular surface of the string when closed and such ability causes the packers to be subject to extrusion. When the ram front packers have been subjected to excessive extrusion and must be replaced, the well pressure must be contained by excessive drilling mud weight and all drillingoperation stopped while the blowout preventer rams are removed and the ram front packers removed and replaced with new packers. This non-drilling "down time" is very expensive. The design of ram packers is thus a compromise to provide the needed feed or available movement of the material with maximum abrasion and extrusion resistance to provide the longest life possible.
Examples of prior ram-type blowout preventers can be seen in U.S. Pat. Nos. 2,883,141 and 3,692,316 which disclose the use of upper and lower anti-extrusion packer plates molded in the packing designed to minimize extrusion damage to the ram front packer. U.S. Pat. No. 2,194,257 discloses a blowout preventer with a ram front packer mounted in a tapered groove in the ram body and having upper and lower tapered anti-extrusion plates embedded in the packing. U.S. Pat. No. 2,237,709 discloses a ram-type blowout preventer with a dovetail groove in the face of the ram to retain a pressure responsive sealing member. U.S. Pat. No. 3,379,255 discloses a well string shearing device with a blind ram blowout preventer for sealing above the cutoff assembly. The ram front packer is composed of resilient material only without upper and lower anti-extrusion plates. The resilient material is not embedded or bonded to the ram, since it must be free to move forward when urged by movement of the stem and plunger head against the thrust plate. The faces of the rams engage when closed to contain the packing. U.S. Pat. No. 1,963,683 discloses a ram-type blowout preventer having segments mounted on screws above and below the packing to slide down responsive to the load of a drill collar to express rubber from the confining channel between the segments and the web of the ram and cause it to flow inwardly and sealingly engage the string.